Before you start pulling things apart, take the LITTLE crankshaft cover off the left side of the engine, its a screw cap about the size of a quarter
put the bike in neutral and see if you can crank the engine over clockwise (on the left side of the bike) with a socket wrench. This will tell you if the engine is seized up and stalling the starter.
It should turn over easy with one hand on the wrench till you start a compression stroke, then it will take a bit of force to turn over till the piston clears top dead center, then be easy to turn till you hit the next compression stroke. If you can turn it at all, the engine is not seized, and if you can go around twice and find both compression strokes, your engine is perfect. BTW, do NOT turn the engine over backwards ( the same direction as the front wheel) the timing chain will go slack - dont do it. If the engine feels like its hitting a CLUNK then dont force it. Nothing should be hitting metal to metal in there.
If the engine is not seized there would be no reason for the starter to be stalling and pulling the battery down to 6.8V, in that case the odds are by 50 to 1 the battery has expired. Since it was dead when you first tried to start it, the battery has sat dead for the last few months, and that is the condition I mentioned in my first post - once it has sat dead for a long time, you can charge it back up, and it will show 12.8V, but it wont have any power to start the bike, because the lead plates are oxidized.
Try jump starting the bike with another battery in parallel, like your car battery, or take the battery out and take it an auto shop that can test it. If the engine is not seized and the battery drops to 6.8V when the starter is engaged, that all points to a weak battery. BTW< Im assuming you had the volt meter probes right on the battery metal tabs, not on the wires connected to the battery. If not check it again. If the battery tabs stay at 12.6V but the voltage on the wires drops to 6.8V when you crank the starter, then the wires to the battery, or the black wire from the battery to the frame is corroded and not able to conduct any current under load.
Last edited by KCW; 02-26-2019 at 09:19 AM.